When you initiate the resizing of an EMR cluster instance group, EMR attempts to add or remove the specified number of instances. When adding instances, EMR launches new instances with the same configuration specified for the initial launch. In some cases, changes made since the cluster was initially launched can prevent new instances of cluster nodes from starting. If EMR is unable to launch new instances when you initiate an increase in the number of instances, EMR changes the status of the cluster instance group to ARRESTED so that you can investigate and address the problem.

Some common causes of this behavior are:

• One of the bootstrap actions used on initial launch is now missing or has been modified, preventing new instances to from successfully executing the bootstrap.
• A change has been made to the associated virtual private cloud (VPC) that affects instance creation. Changes to the VPC network routing, security groups, or access control lists (ACLs) can impact the creation of new cluster instances.
• A change has been made to IAM permissions for the roles used by the cluster instance group.